1. Field of the Invention
The present invention relates to an aluminum die-cast material for boat equipment generally required to have seawater corrosion resistance and mechanical strength, especially for outboard and inboard propelling apparatuses of boats and water-jet pumps and other members constituting parts of boat bottoms.
2. Description of the Related Art
The recent tendency in the art is toward weight-reduced boat equipment, for which are being much used aluminum die-cast materials. Some aluminum die-cast materials are defined in JIS H-5302 (1990) xe2x80x9cAluminum Alloy Die-Castsxe2x80x9d, including ADC1 (Cu: at most 1.0% by weight), ADC3 (Cu: at most 0.6% by weight), ADC5 (Cu: at most 0.2% by weight), ADC6 (Cu: at most 0.1% by weight), ADC10, 10Z (Cu: 2 to 4% by weight), ADC12, 12Z (Cu: 1.5 to 3.5% by weight), and ADC14 (Cu: 4 to 5% by weight).
The constituent Cu in such aluminum die-cast materials is a primary cause of rust. Therefore, the Cu content of aluminum die-cast materials for boats must be at most 0.6% by weight for their seawater corrosion resistance. ADC3 (Cu: at most 0.6% by weight), ADC5 (Cu: at most 0.2% by weight) and ADC6 (Cu: at most 0.1% by weight) will meet the requirement.
On the other hand, the fluidity of melts of such die-cast materials is another important factor for smoothly die-casting them. The fluidity of their melts essentially depends on the constituent Si therein. In this respect, the three materials of ADC3 (Si: 9 to 10% by weight), ADC5 (Si: at most 0.3% by weight) and ADC6 (Si: at most 1.0% by weight) are discussed. Their fluidity is in the order of ADC5 less than ADC6 less than ADC3. That is, the fluidity of melts of ADC5 and ADC6 is not so high.
ADC3 could have good fluidity and good corrosion resistance but its mechanical strength is lower than that of ADC12.
The present invention has been achieved with the above matters taken into consideration, and its object is therefore to provide a novel aluminum die-cast material that satisfies all the requirements of fluidity, corrosion resistance and mechanical strength.
According to an aspect of the present invention, there is provided an aluminum die-cast material for boats, which consists essentially of 0.15% by weight or less of Cu (copper), 10.0 to 11.5% by weight of Si (silicon), 1.0 to 2.5% by weight of Mg (magnesium), 0.15% by weight or less of Zn (zinc), 0.7 to 0.9% by weight of Fe (iron), 0.4 to 0.6% by weight of Mn (manganese), 0.1% by weight or less of Ni (nickel) and 0.1% by weight or less of Sn (tin), and the balance of Al (aluminum).
Cu-containing aluminum alloys having a Cu content of larger than 0.6% by weight are seriously corroded with seawater. Therefore, for its seawater corrosion resistance, the Cu content of the aluminum die-cast material of the invention must be limited to at most 0.6% by weight. In case where its Cu content is not larger than 0.15% by weight, the material provides better corrosion resistance. Therefore, the Cu content of the material is preferably at most 0.15% by weight.
For better fluidity in casting, a higher Si content is preferred. For providing fluidity comparable to or higher than that of ADC3 (Si: 9 to 10% by weight) which is said to have good fluidity, the Si content of the material of the invention is limited to at least 10% by weight. However, if larger than 11.5% by weight, too much Si will give primary crystals whereby the material will be brittle and its strength is lowered. Therefore, it is desirable that the Si content of the material falls in a range of 10.0 to 11.5% by weight.
The tensile strength of ADC3 is 245 N/mm2. It is desirable that the tensile strength of the material of the invention is not lower than that of ADC3. When quenched in water, the material having an Mg content of at least 1.0% by weight could have a tensile strength of at least 245 N/mm2, and it is good. However, if larger than 2.5% by weight, it is undesirable since too much Mg will increase the viscosity of molten aluminum (aluminum melt) whereby the fluidity of the aluminum melt is lowered. Accordingly, it is desirable that the Mg content of the material of the invention falls between 1.0 and 2.5% by weight.
Preferably, Zn is not in the material as it lowers the corrosion resistance of Al. However, Zn, if any, in the material will improve the workability of the material and will therefore lower the production costs thereof. Accordingly, the material may contain at most 0.15% by weight of Zn.
An aluminum melt having an Fe content of smaller than 0.7% by weight will stick on the surface of a mold in which it is cast, and therefore cannot be smoothly and continuously cast owing to its negative influence on the mold. Aluminum alloys having an Fe content of larger than 0.9% by weight are brittle and their mechanical properties are poor. Accordingly, it is desirable that the Fe content of the material of the invention falls between 0.7 and 0.9% by weight.
Adding Mn to the material is effective for preventing the negative influence of iron compounds on the material, as Mn forms a tabular intermetallic compound of Al-Mn(Fe)-Si in the material. The intermetallic compound retards the negative influence of Fe on the material, thereby preventing the toughness and the impact resistance of the material from being lowered. If smaller than 0.4% by weight, Mn will be ineffective; but if larger than 0.6% by weight, too much Mn will crystallize to lower the mechanical properties of the material. Accordingly, it is desirable that the Mn content of the material of the invention falls in a range of 0.4 and 0.6% by weight.
Preferably, Ni is not in the material as it seriously lowers the corrosion resistance of Al. However, reducing too much the Ni content of the material is unfavorable as the production costs of the material will increase. In view of the balance between the corrosion resistance and the production costs of the material, it is desirable that the Ni content of the material is at most 0.1% by weight.
Preferably, Sn is not also in the material as it seriously lowers the corrosion resistance of Al. However, reducing too much the Sn content of the material is also unfavorable as the production costs of the material will increase. In view of the balance of the corrosion resistance and the production costs of the material, it is desirable that the Sn content of the material is at most 0.1% by weight.